Method for producing porous metal parts having uniform fluid permeability



Nov. 25,1969 D. A. VOORHIES METHOD FOR PRODUCING POROUS METAL PARTSHAVING UNIFORM FLUID PERMEABIL-ITY Filed Nov.. 1.5, 1966 lJ/ZJ INVEN'IUAA ORNEY United States Patent US. Cl. 204 -141 5 Claims ABSTRACT OF THEDISCLOSURE Precise control over the permeability of a porous metal partis accomplished by monitoring the flow rate of the air through the partduring electrolytic erosion of metal from the surface pores. Theelectrolytic erosion is automatically discontinued when the backpressure of the air falls below a predetermined value.

This invention relates to the production of porous metal parts such asfluid bearing bushings and, more particularly, to a method of impartinga desired permeability to a part following a machining process.

Porous metal parts, such as bushings for air bearings, are usuallyproduced by compacting, sintering and sizing powdered metals into thedesired shape. The dimensional accuracy of porous metal parts soproduced is not sufiicient for many uses. Therefore it is oftendesirable or necessary to machine, i.e., grind or ream, a surface of thepart. This machining tends to fill the surface pores with metal thusclosing the pores and materially reducing the fluid permeability of thepart.

In accordance with the subject invention, precise control over thepermeability of a porous metal part may be accomplished in such afashion that the permeability of the part may be increased to a desiredvalue following a process such as machining which closes surface pores.In general, this is accomplished by electrolytically eroding metal fromthe surface pores while applying air or other fluid under pressure tothe part. An indication that the desired permeability has been reachedmay be accomplished by monitoring the rate of fluid flow through theporous part. When the desired permeability is reached, the electrolyticerosion is abruptly discontinued leaving the part in the desired state.

It is to be understood that while the subject invention is especiallyuseful in restoring permeability decreased by surface machining, it isapplicable to all situations wherein it is desired to increase thepermeability of a porous metal part by electrolytically eroding metalfrom a surface thereof.

A method consonant with the invention as well as apparatus for carryingout the method is specifically described in the following text withreference to the drawing of which the single figure illustrates theapparatus.

Referring to the figure, which will serve to illustrate an applicationof the invention, is a cylindrical porous metal bushing. The outersurface of bushing 10 has been machined by grinding or reaming to adesired dimension. In the process of grinding the bushing 10, the poreson the outer cylindrical surface have been filled with metal therebyreducing the permeability of the bushing to an undesirably low value.Before the bushing 10 can be used in an air bearing, it is thusnecessary to improve the permeability thereof by removing the metal fromthe surface pores.

To accomplish this in accordance with the invention, the bushing 10 ismounted on a conductive support means generally designated at 12 anddisposed within a container 14 which is substantially filled with anelectrolyte 3,480,530 Patented Nov. 25, 1969 16. A hollow cylindricalelectrode is disposed within the container 14 to surround the bushing 10in radially spaced relationship therewith. The electrolyte completelycovers both the bushing 10 and the electrode 18. Electrode 18 sits on anannular insulator 20.

Energization of the electrolytic erosion circuit is accomplished byconductors 22 and 24 which are connected to a power source indicated bylegend. Conductor 22 is connected to the cylindrical insulator 18.Conductor 24 is electrically connected to the support means 12. Theconductive path fromlthe source to the support 12, however, includes inseries relationship therewith a normally closed pressure switch 26, thefunction of which is described in further detail below.

Assuming the switch 26 to be closed, current passes through the switch26, the conductor 24, the support means 12, the bushing 10, electrolyte16, electrode 18, and conductor 22 back to the source. Bushing 10 whichthus constitutes the anode in the DC circuit just described iselectrolytically eroded so that the metal which closes the surface poresdue to the machining process is gradually removed. An electrolyte 16which has no strong chemical reaction with the metal bushing 10 ispreferred.

For the purpose of ascertaining the point in time in which thepermeability of bushing 10 has increased to the desired value, asuitable fluid such as air is admitted under pressure to the surface ofan inner bore of the bushing 10. The air is supplied by a main supplyduct 28 and a branch duct 30 which contains a restrictive 32. Duct 30 isconnected to a chuck assembly which includes a conductive body member 34and a threaded clamp 36. Body member 34 is fastened to a support board38 by means of a flared annular insulator 40 and is fitted to receive aconductive pipe 42 which forms part of the bushing support assembly 12.The conductive pipe 42 is inserted into the inner bore of bushing 10 soas to sealingly engage the inner surface of the bushing. Pipe 42 isprovided with openings not shown which permit air to be suppliedsubstantially uniformally to the inner surface of bushing 10. This airpasses through the bushing 10 due to the porosity thereof and escapesthrough the electrolyte 16.

As described above, the machining process previously performed on theouter surface of the cylindrical bushing 10 tends to close the surfacepores thereof. Therefore, the permeability of the bushing 10 is reducedand the air which is permitted to flow through the bushing 10 issubstantially reduced, if not completely out 01f. Since the bushing 10is contemplated for use in a combination such as an air bearing where apredetermined permeability is necessary, the aforementioned electrolyticerosion process is carried out to improve or increase the permeabilityof bushing 10. The desired permeability is indicated as a function offlow rate of the presurized air through the porous bushing 10. Tomonitor this flow rate, the branch duct 30 is connected by means of asection 44 to the pressure sensitive switch 26. The switch 26, which isnormally closed, may be adjusted by means indicated at 46 to openwhenever the air pressure exerted upon a diaphragm member (not shown)falls below a predetermined value. As the electrolytic erosion processcontinues, the flow rate of air through bushing 10 rises, thusdecreasing the back pressure in branch duct 30. When the desired bushingpermeability is attained, switch 26 opens, cutting off the flow ofelectric current in the electrolytic erosion circuit. Opening thecurrent supply circuit abruptly terminates the erosion process, leavingthe bushing 10 in the desired state of permeability.

As previously mentioned, the utility of the method described is notlimited to porous metal parts the permeability of which has been reducedby a machining process. The invention may be advantageously applied toincreasing the permeability of any porous metal part by enlarging thepores in the part adjacent the surface thereof. Similarly, the processdescribed above may be applied to porous parts of various geometries bysuitable adjustment of the air inlet pipe 42 and the electrode 18.Therefore, this description is not to be construed in a limiting sense.For a definition of the invention, reference should be had to theappended claims.

What is claimed is:

1. A method of increasing the permeability of a porous metal partcomprising the steps of electrolytically eroding metal from the pores inat least one surface of the part, simultaneously applying fluid underpressure to another surface of the part, monitoring the flow rate of thefluid applied to the part during the electrolytic erosion, anddiscontinuing the erosion when the desired fiow rate is reached.

2. The method defined in claim 1 wherein the electrolytic erosion isaccomplished by immersing the part in an electrolyte and causingelectric current to flow between the part and an electrode adjacent butspaced from said one surface of the part.

3. The method defined in claim 2 wherein the step of discontinuing isaccomplished by terminating said flow of electric current whenever adesired flow rate is reached. 4. The method defined in claim 1 whereinthe fluid 1s air.

' 5. The method defined in claim 1 wherein said monitoring the flow rateis accomplished by measuring the back pressure exerted by the fiuid as aresult of restricted flow through the porous part.

References Cited UNITED STATES PATENTS 3,220,937 11/1965 Friese et al.204-284 US. Cl. X.R. 204-229

